1. Field of the Invention
The present invention relates to a high frequency oscillator (HFO) ventilator of the type in which high frequency pressure oscillations around a mean airway pressure are induced in a respiration gas to move small volumes of gas in to and out of a patient""s airways in order to provide full ventilation support.
2. Description of the Prior Art
For HFO ventilation, it is known to provide an oscillator unit for alternately supplying a as volume to and removing a gas volume from a proximal end of a gas conduit (oscillator volume) at a predetermined high frequency, the gas conduit having a proximal end connectable to the oscillator unit and a distal end connectable with the patient""s airways and, between the proximal end and the distal end of the conduit, to provide an inlet for bias gas and an outlet through which gas is removed from the conduit. The outlet typically includes a variable flow resistance which, together with a suitable flow rate of bias gas, establishes a mean airway pressure for gas within the conduit which is selected to open the lungs sufficiently to achieve an optimum gas transfer. The gas pressure within the conduit is then caused to oscillate around this mean by the alternate supply and removal of gas by the oscillator unit. This causes a volume of gas to move through the distal end of the conduit and be alternately supplied to (inspiration tidal volume) and removed from (expiration tidal volume) the patient""s airways.
A problem with such a known HFO ventilator is that it is relatively inflexible, since the inspiration and expiration tidal volumes are dependent on the oscillator volume. The oscillator volume therefore must be changed in order to change either of the tidal volumes. It is difficult to provide an oscillator unit in which the oscillation can may be varied with any great precision or speed.
In addition to establishing the mean airway pressure, the bias gas flow also flushes carbon dioxide (CO2) through the outlet that is expired by the patient and drawn into the conduit as part of the expiration tidal volume, so as to prevent the CO2 from being re-breathed. The rate at which the CO2 is removed depends on the flow rate of bias gas through the conduit, from inlet to outlet.
A further problem with the known HFO ventilator is that in order to provide a variable bias gas flow rate the oscillator unit must be capable of providing a variable oscillator volume greater than the desired inspiration tidal volume. This is because as the bias flow rate increases the flow resistance at the outlet must be decreased in order to maintain a predetermined mean airway pressure. This results in a greater amount of the oscillator volume flowing through the outlet since its resistance is lowered compared with that at the distal end of the conduit. In order to maintain a desired inspiration tidal volume, the oscillator volume must be increased to exceed this inspiration tidal volume.
An object of the present invention is to provide an HFO ventilator which allows the inspiration tidal volume to be adjusted independently of the oscillator volume.
The above object is achieved in accordance with the principles of the present invention in an HFO ventilator of the type described above, which additionally includes a flow controller which divides the oscillator volume between the distal and the conduit and the outlet, thereby allowing the inspiration title volume to be selectively set, or the ratio of the inspiration title volume to the expiration title volume to be selectively varied.
The flow controller can be a size-variable restriction disposed within the conduit at a location between the inlet and outlet and the distal end of the conduit, for example in a common stem of a Y-piece connector, and which is operable to regulate the resistance to gas flow to and from the distal end of the conduit as the size is varied. Being located within the conduit enables the controller to be provided without significantly increasing the deadspace of the ventilator.
The HFO ventilator can have an oscillator unit that is capable of providing gross changes in the oscillator volume, for example providing a different maximum oscillator volume for use in ventilating different types of patients, such as adults, pediatric patients and neonates. In this embodiment the flow controller varies the tidal volume to suit the ventilatory requirements of individual patients within each category. This extends the operational range of the HFO ventilator without the need for accurate, and hence relatively expensive, oscillator volume control equipment.